Anything and everything that is piled. Which actually excludes nuclear reactors, save for certain experimental types that haven't been in use for nearly a century.

Baffle plates are used nowadays in the construction of nuclear reactors.

...But would be incompatible with piles, as something that is piled cannot slosh around.

Having an open circuit in a nuclear reactor is a matter of grave concern, even nowadays.

It's nothing specific to nuclear reactors or powerplants, though. And it's a bit unlikely to survive as a concept as power transfer technology mutates; it would be meaningless in microwave transfer of energy, say.

Timo, you can dance around the terms, but the basic fact is this: The people who wrote these episodes had a basic understanding of nuclear reactors.

Sure. And then they threw around the terminology to create a meaningless mess...

...Which isn't necessarily a bad thing. After all, it allows us to interpret the terminology any way we like to.